Diel, seasonal and vertical changes in the abundance, biomass and community structure of pelagic polychaetes at the subtropical station S1 in the western North Pacific: comparison with the results from the subarctic station K2

Abstract Information on pelagic polychaete community structure in the western North Pacific is available for the subarctic region (Station K2) but not for the subtropical region. Hence, we analyzed day–night vertically stratified samples collected in eight layers within the first 1000 m of the water column during four seasons in 1 year, using the same sampling method as St. K2, at the subtropical region (Station S1). At St. S1, 27 species of pelagic polychaetes belonging to 13 genera and six families were identified. The annual mean abundance was 35.0 ind. 1000 m−3 and the biomass was 17.3 mg WW 1000 m−3. At St. S1, the numbers of genera and species were higher and the annual mean abundance and biomasses were much lower than St. K2. The pelagic polychaetes often peaked in the mesopelagic layer at St. K2, with the carnivores and particle feeders peaking in the epipelagic and mesopelagic layers, respectively. At St.S1, the carnivorous species predominated throughout the entire water column, and were most abundant in the epipelagic layer. Thus, In the western Pacific Ocean, the subarctic pelagic polychaete community structure changed vertically with feeding ecology. On the other hand, the subtropical community may be adapted to conditions of high irradiance and light transmission.

Information on pelagic polychaete community structure in the western North Pacific is available for the subarctic region (Station K2) but not for the subtropical region.Hence, we analyzed day-night vertically stratified samples collected in eight layers within the first 1000 m of the water column during four seasons in 1 year, using the same sampling method as St. K2, at the subtropical region (Station S1).At St. S1, 27 species of pelagic polychaetes belonging to 13 genera and six families were identified.The annual mean abundance was 35.0 ind.1000 m −3 and the biomass was 17.3 mg WW 1000 m −3 .At St. S1, the numbers of genera and species were higher and the annual mean abundance and biomasses were much lower than St. K2.The pelagic polychaetes often peaked in the mesopelagic layer at St. K2, with the carnivores and particle feeders peaking in the epipelagic and mesopelagic layers, respectively.

INTRODUCTION
Pelagic polychaetes are widely distributed from the surface to the deep layers of oceans worldwide (Halanych et al., 2007;Ushakov, 1974).Pelagic polychaete diets differs widely among species: herbivorous (Day, 1967;Hopkins and Torres, 1989), carnivorous (Feigenbaum, 1979;Lebour, 1922;Rakusa-Suszczewski, 1968) and detritivorous (Christiansen et al., 2018;Robison et al., 2010;Uttal and Buck, 1996).Thus, understanding accurately the role of pelagic polychaete in food webs and vertical material flux of marine ecosystems worldwide requires information at the species level.However, our knowledge of the ecology of pelagic polychaetes, especially their community structure, is scarce compared to their importance.
Regarding pelagic polychaetes in the Pacific Ocean, regionally different distributions of genera and species have been reported, especially between the subarctic and subtropical regions (Tebble, 1962).The subarcticsubtropical comparisons have been carried out by analyzing zooplankton swimmer samples, known to swim actively (not just sink), collected by sediment traps moored at a depth of 200 m (Amei et al., 2020), reporting seasonal abundances (fluxes) and population structures of the dominant species.Time-series surveys are essential to reveal the effect of environmental factors on the community structure of pelagic polychaete; however, no study has compared the time-series changes in the vertical structure between subarctic and subtropical regions.Regarding the community structure of pelagic polychaetes at St. K2 located in the western subarctic Pacific, diel, vertical and seasonal changes in community structure have been reported based on the analysis of day-night vertical stratified samples collected from the 0-1000 m water column in four seasons covering 1 year (Amei et al., 2021).These findings are important for comprehensively evaluating diel, vertical and seasonal changes in pelagic polychaete communities from the surface to the deep sea.However, there are few comparable studies from other regions.At St. S1, JAMSTEC's subtropical time-series station, zooplankton samples were collected using the same methods as at St. K2 (Kitamura et al., 2016).However, the polychaete abundance, biomass and community structure at St. S1 have not yet been analyzed.
Hence, in this study, we analyzed pelagic polychaetes in the samples collected by day-night vertically stratified sampling to a depth of 1000 m during four seasons in 1 year at St. S1 in the western Pacific Ocean.We compared the results of this study with those reported in the study by Amei et al. at St. K2 in the western Pacific Ocean (Amei et al., 2021), who applied the same methods as those applied in this study.This comparison may reveal the regional differences in polychaete communities between the subarctic and subtropical regions of the western North Pacific Ocean and the factors affecting them.
Based on temperature and salinity, the water masses were identified at each depth and sampling date.Throughout the year, we classified five water masses at St. S1: seasonal pycnocline water (SPW), subtropical mode water (STMW), transition water (TW), upper North Pacific intermediate water (upper NPIW) and lower North Pacific intermediate water (lower NPIW), based on the T-S characteristics at each sampling layer (Masuzawa, 1969;Yasuda, 2003;Yasuda et al., 2001).SPW is the water above the pycnocline and is thus susceptible to solar radiation.STMW corresponds to the water of the remnant winter mixed layer in the subtropical North Pacific and has a temperature of 16-18 • C ∼100-300 m depth (Masuzawa, 1969).The water between STMW and NPIW was defined as TW in this study.A salinity minimum near 26.8 σ θ characterizes NPIW.This water originates from the Okhotsk Sea and the western subarctic gyre (WSG) and flows toward the Kuroshio extension region (Yasuda, 2003).The upper NPIW (density centered at 26.6 σ θ-26.9 σ θ) and lower NPIW (density centered at 27.0 σ θ-27.4 σ θ) were more strongly affected by the water in the Okhotsk Sea and WSG, respectively (Yasuda et al., 2001).
We sorted pelagic polychaetes from 1/4th of the bottle samples collected at each depth layer in the land laboratory.The specimens were identified to species and counted under a stereomicroscope.Species identification was performed to the lowest level possible, following Dales (1957) and Tebble (1962).For each genus or species, a batch of specimens in each sample was placed on a preweighed mesh, and the water was removed with the aid of wiping paper.Then, the wet weight (WW) was measured using an electronic balance (Mettler Toledo AT261) with a precision of 0.01 mg.The vertical distributions of the total abundance (ind.1000 m −3 ) and total WW (mg 1000 WW m −3 ) were then quantified.For each sample, the Shannon-Weaver diversity index (H ) was calculated according to abundance and biomass using the following equation: Where S is the number of species, and p i is the relative dominance of species i (Shannon and Weaver, 1949).
To determine the community structure of pelagic polychaetes, we calculated the similarity of samples based on abundance using the Bray-Curtis method (Bray and Curtis, 1957).The abundance data of pelagic polychaete species (X : ind.1000 m −3 ) were log-transformed (log [X + 1]) before analysis.A cluster analysis connected to complete linkage methods was performed for sample grouping.Nonmetric multidimensional scaling (NMDS; Minchin, 1987) and Pearson's multidimensional coefficient analysis analyzed the relationships between environmental variables (depth, temperature, salinity, DO and fluorescence) and polychaete sample ordinations.
To evaluate the effects of the seasons, day-night cycles and depth on the similarities between the samples, the permutational multivariate analysis of variance (PER-MANOVA), a non-parametric multivariate statistical permutation test, was performed; these analyses were performed using PRIMER v7 (PRIMER-E, Ltd).Intercommunity and water mass differences in the abundance of each species were tested using one-way ANOVA and the Tukey-Kramer test; these analyses were performed using STAT-View.

Hydrography
The vertical changes in temperature, salinity, DO and fluorescence for each sampling date at St. S1 are shown in Fig. 2. Throughout the study period, the temperature, salinity, DO and fluorescence ranged from 3.5-26.8• C, 34.0-34.9,0.8-5.4mL L −1 and 0.02-0.91mg m −3 , respectively.Regarding the temperature, the seasonal thermocline developed at ∼50 m in November and July.In February and April, the temperature was almost uniform at 200-300 m, decreased gradually at 100-300 m and rapidly since 300-600 m and then smoothly decreased by 2-5 • C at depths below 600 m.The salinity reached a maximum at ∼50 m in November and July.Throughout the year, the salinity decreased gradually with depth between 100 and 300 m, reaching the minimum (ca.34.0) level at ∼600-700 m, and showed an increase with increasing depths beyond 700 m.The DO decreased with increasing depth.The rate of decrease in the DO was high at depths below 600 m.An oxygen minimum layer (OML) of < 2 mL L −1 ) was observed below 800 m.The fluorescence was high at depths of 0-200 m throughout the year.In February, the fluorescence was high at depths of 0-100 m.In other

Pelagic polychaete abundance and biomass
We found 27 pelagic polychaete species belonging to 13 genera and six families at St. S1 (Table II).The polychaete group with large eyes, previously categorized as tribe Alciopini (Phyllodocidae), was highly diverse (14 species).
The most abundant species was the Tomopteris spp.without tails (annual mean: 21 ind.1000 m −3 ), which accounted for 59% of the total abundance.For species identification of Tomopteris spp.(Tomopteridae), the following three characteristics were the most important: (i) the presence or absence of first tentacular cirri between the horn-like antenna and cirri-like appendages, (ii) the presence or absence and position of glands in the rami and (iii) the presence or absence of a tail on the pygidium.Because the rami condition was not good in this study and we could not observe glands in the rami (chromophile glands, hyaline glands and rosette glands), we described Tomopteris species as Tomopteris spp.(without tail), Tomopteris sp.1 (without tail) and Tomopteris spp.(with tail).The abundance of pelagic polychaetes in the water column ranged from 1 to 260 ind.1000 m −3 (Fig. 4).Throughout the days and nights of all seasons, the highest abundance was observed at depths of 0-200 m, with the maximum reported in November.
In terms of biomass, the Tomopteris spp.without a tail were also dominant, but to a limited extent (18% of the total pelagic polychaete biomass) (Table II).The biomass composition of the large Tomopteris spp. with a tail was high (12%), while their abundance was low (1.9%).
The WW biomass ranged between 0.1 and 83.2 mg WW 1000 m −3 (Fig. 5).Increased biomass in the daytime was observed at 0-200 m for all three seasons, The species diversity (H ) of the pelagic polychaetes in the layers was 0-1.5 (based on abundance) and 0-1.6 (based on biomass), and the maximum H was usually observed in the surface layer with no evident seasonal patterns (Figs 4 and 5).The H was 0.47-1.01,based on the abundance in the samples.
A comparison of species abundance in each water mass showed that there was no significant difference in the abundances of species between different water masses, except for Vanadis formosa and Sagitella kowalewskii (Table III) that were abundant in the surface water mass, SPW and STMW (P < 0.05).Fifteen species (12 species belonging to Alciopini [tribe]) were distributed only at SPW or STMW, although not at a significant level.

Pelagic polychaete community structure
Based on abundance, the pelagic polychaete community was clustered into six community groups (A-F) with a 45% dissimilarity (Fig. 6a).Each group contained 2-22 samples.NMDS separated the groups, and various environmental factors (depth, salinity, DO and fluorescence) were found to have significant directions according to Pearson's multidimensional coefficient.From the plotted directions, depth and DO were related to pelagic polychaete clustering.Depth and environmental factors depending on the depth (salinity, temperature and fluorescence) affect horizontally, and the clustering groups are plotted separately in the horizontal direction.DO had a significant effect on the grouping of groups A (high DO), B (moderate DO) and C (low DO) (Fig. 6b).The mean abundance of group A (43.9 ind.1000 m −3 ) was higher than that of groups B (18.3 ind.1000 m −3 ) and C (9.1 ind.1000 m −3 ), with Tomopteris spp.(without tail) accounting for 56%.Tomopteris spp.(without tail) was dominant in group B, while Sagitella spp. was dominant in group C (Fig. 6c).Low abundances were observed in groups D, E and F. Different species compositions were observed in these groups.Typhloscolex muelleri was dominant in group D, Pelagobia longicirrata, in group E and Travisiopsis spp., only in group F. The numbers of genera differed within the groups (3-11 genera), and some genera occurred in specific groups (Alciopina spp.: A-C, Plotohelmis spp.: B, Rhynchonereella spp.: A and B, Sagitella spp.: A-C, and Torrea spp.: A) (Fig. 6c).Most of them (except Sagitella spp.) belonged to the Alciopini (tribe).Inter-group differences in abundance were detected only for V .formosa, which was abundant in group B (P < 0.05, Table IV).
The diel, vertical and seasonal distributions of pelagic polychaete community groups at St. S1 are shown in Fig. 7.The occurrence of each group was governed by depth.The high-abundance groups (A and B) were observed at depths of 0-300 m.Group C was mainly observed at depths of 200-750 m.The occurrence of low-abundance groups (D, E and F) was restricted to depths below 300 m.Thus, the distribution of the pelagic polychaete community primarily varied vertically and did not show diel or seasonal patterns.The results of the PERMANOVA also indicated that depth was the most prominent environmental factor in polychaete grouping (P < 0.001) (Table V).The interaction between season×day/night also had a significant effect (P < 0.05).

Fauna and species diversity
In the western North Pacific, the number of pelagic polychaete species at subtropical St. S1 (27 species) (Table II) was much higher than that at subarctic St. K2 (10 species) (Amei et al., 2021).Four common families were present in both regions: Lopadorrhynchidae, Phyllodocidae, Typhloscolecidae and Tomopteridae.Iospilidae and Flabelligeridae occurred only in the subarctic region, whereas Amphinomidae and Polynoidae were present only in the subtropical region.Fernández-Álamo et al. (2003) reported a clear latitudinal pattern reported for the 0-200 m water column between 21 • N and 40  Fernández-Álamo et al., 2003).Among the three groups, a south group characterized by the presence of subtropical pelagic polychaete genera such as Vanadis, Rhynchonerella and Plotohelmis had the most similar community with that of St. S1 reported in this study.The high diversity shown in St. S1 suggests the presence of a difference in pelagic polychaete community structure between the west and east region of the subtropical gyre in the Pacific Ocean.Tebble (1962) reported seven tomopterid species from a wide region of the Pacific Ocean, including the sampling region of this study.He also mentioned the important morphologic traits for Pacific tomopterid identification.According to him, [Tomopteris spp.with tail] includes Tomopteris nisseni, Tomopteris krampi, Tomopteris apsteini, Tomopteris pacifica, and [Tomopteris spp.without tail] includes Tomopteris planktonis, Tomopteris elegans, Tomopteris ligulata and Tomopteris septentrionalis.In subtropical S1, Tomopteris spp.(with and without tail), with high proportions throughout the water column, were concentrated at the surface layer.Tomopteridae, a common family worldwide, is reported to dominate the Pacific Ocean, especially in subtropical and subarctic regions (e.g.Tebble, 1962).In the present study, subtropical Tomopteris spp.(without tail) were too small to preserve the rami.In both regions, this family was distributed widely at 0-1000 m, and peaks in its abundance were observed at depths shallower than 300 m.Tomopteris spp.(without tail) at S1 showed no seasonal changes in vertical distribution (Fig. 8), in contrast to T. septentrionalis at K2. Unfortunately, in this study, we cannot treat Tomopteris spp.(without tail) as one species.However, the difference in the vertical distribution between these subtropical and subarctic dominant Tomopteris species suggests that Tomopteris species have different dynamics depending on marine environmental factors, such as the concentration of chlorophyll a in each region.
Within the four families commonly occurring in both regions, the Alciopini (tribe), belonging to Phyllodocidae, showed substantial diversity in the subtropical region.Thus, while only two species were observed in the subarctic region, 14 were in the subtropical region.Phyllodocidae, including Alciopini, was the second most dominant family in the subtropical region (Table II).Alciopid polychaetes are abundant and adapted to tropical/subtropical regions with eyes equipped with morphologically specialized lenses (Dales, 1957;Ushakov, 1974).The eyes of alciopid polychaetes have accessory retinas beside the lens that can even detect ultraviolet light (Wald and Rayport, 1977).These facts suggest that   alciopids are well adapted to high light conditions under low light attenuation in the oligotrophic tropical/subtropical region.Thus, active visual-predator alciopids may have more opportunities to prey under sufficient light with high light radiation and transmission in the subtropical region than in the subarctic region.These characteristics of alciopids (high swimming ability and specialized eyes) may allow for their high diversity and numerical dominance in the pelagic polychaete community in the subtropical region.Pelagobia longicirrata was a common species in both regions.Pelagobia longicirrata has been treated as a cosmopolitan species; however, Kolbasova and Neretina (2021) reported that P. longicirrata has some geographically different clades in a phylogenetic tree based 28S and 18S.

Table IV: Comparison of the polychaete abundances in the six groups (A-F) identified by
There is no phylogenetic information on P. longicirrata in the Pacific Ocean.In the subtropical region, P. longicirrata accounted for only 2% of the abundance and was vertically concentrated at depths greater than 750 m (Table II).This species is the most abundant in the subarctic region, accounting for 74% of the pelagic polychaete abundance (Amei et al., 2021).In both regions, P. longicirrata was distributed at depths greater than 150 m.They showed distribution at a wide depth range (150-1 000 m) at subarctic St. K2 and were concentrated to the deepest layer (750-1000 m) at subtropical St. S1.Thus, the distribution of P. longiciratta ranged from the subarctic region (below 150 m) to the subtropical region (in the NPIW layer).The abundance and biomass of P. longicirrata in the subtropical region were much lower  than those in the subarctic region.It may be related to the lower primary production in the subtropical region than in the subarctic region.For various zooplankton taxa, higher species diversity has been reported in the subtropical regions than in the subarctic areas (e.g.Tittensor et al., 2010;Sun and Wang, 2017).Sun and Wang (2017) showed the change of diversity H on the transect of 160 • E from 4 • S to 46 • N based on zooplanktons sampled by 200 μm-mesh twoparalleled WP2 net from 200 m depth.They reported a rapid decrease of H between 30 • N and 40 • N.However, the H values of the pelagic polychaetes throughout each layer were similar (0.82-1.17) at K2 and (0.47-1.01) at S1.While the H values were similar for both regions, it should be noted that the vertical distribution of H varied with the region.Thus, H peaked at 100-300 m at St. K2, while peaks formed at the surface layer at St. S1 (Fig. 9).These results reflect regional differences in pelagic polychaete communities.At St. K2, T. septentrionalis and T. muelleri dominated at 0-100 m, P. longicirrata dominated at the deeper layers below 300 m, and the H peaked at 100-300 m depth where all the species occurred (Fig. 9).At St. S1, the vertical distribution of pelagic polychaetes was concentrated in the surface layer.The H values also peaked near the surface layer (Fig. 9).At both stations, vertical patterns of H were similar to that of pelagic polychaete abundance.It suggests that the diversity of rare taxa pelagic polychaetes strongly depends on their abundance.

Abundance and biomass
Annual mean abundance and biomass of the pelagic polychaetes in the subtropical region were 35.0 ind.1000 m −3 and 17.3 mg WW 1000 m −3 , respectively (Table II).These values are much lower than those in the subarctic region (239.2ind.1000 m −3 and 517.5 mg WW 1000 m −3 ; Amei et al., 2021) (Fig. 9).The differences between the regions (S1:K2) were expressed as a factor of 0.146 (abundance) or 0.033 (biomass).The vertical distributions of abundance also varied with the region.The vertical patterns of pelagic polychaete abundance at St. K2 resembled a gradual arch-shape with peaks from 200 to 500 m depth (Amei et al., 2021).On the other hand, that at St. S1 decreased increasing depth.For comparison of the vertical pattern of pelagic polychaete community structures between the subtropical and subarctic Pacific Ocean, we listed a diel-mean abundance of pelagic  polychaetes for each depth layer shown by this study and comparable previous studies (Table VI).Steinberg et al. (2008) reported vertical patterns of some zooplankton taxa at subtropical St. ALOHA and subarctic St. K2, and Mileykovskiy (1969) reported the on pelagic polychaetes at Kuril Kamchatka Trench.Excluding Mileykovskiy (1969), all studies used IONESS or MOCKNESS with 335 μm for sampling.The percentages of distributing depth of pelagic polychaetes are similar between this study and the previous studies (Fig. 10).Kitamura et al. (2016), reporting biomass data of mesozooplankton in the same samples as this study, suggested the occurrence of diel vertical migration (DVM) and seasonality at St. S1 related to the primary production.However, pelagic polychaetes at St. S1 did not show significant DVM and seasonality.A pelagic polychaete DVM cannot be detected because of the surface layer limited distribution of pelagic polychaetes in the subtropical Pacific Ocean, especially Alciopini.It is also possible that pelagic polychaete DVM was too small to detect by the resolution of sampling layers in this study.Besides, the seasonality of this study's pelagic polychaetes of St. S1 can be related to their reproduction and ecology.However, almost all of them remain unknown, especially for subtropical species.Given the regional differences in mesozooplankton stocks in the western North Pacific, the annual mean biomass of the whole zooplankton population at St. K2 (5.8 mg C m −2 ) has been reported to be ten times higher than that at St. S1 (0.52 mg C m −2 ) (Kitamura et al., 2016).Further, the zooplankton biomass at 0-1 000 m depths was greater at St. K2 than at the subtropical St. ALOHA station (Steinberg et al., 2008).Based on the contents of the sediment trap moored at 200 m depth at St. K2 and S1, zooplankton fluxes in number and mass have also been reported to be 2.3-6.5 times higher at St. K2 than at St. S1 (Yokoi et al., 2018).The flux of pelagic polychaetes was also analyzed for these sediment trap samples, and their annual mean values were 5.37 ind.m −2 day −1 at St. K2 and 1.36 ind.m −2 day −1 at St. S1 (Amei et al., 2020).Thus, the abundance and biomass were several times greater in the subarctic region than in the subtropical area, which are common phenomena for pelagic polychaetes and whole zooplankton.
In the K2S1 project, which focused on regional comparisons of carbon cycles, the standing stocks (mg C m −2 ) and fluxes (mg C m −2 day −1 ) of various biogeochemical components have been quantified (Honda et al., 2016(Honda et al., , 2017)).While the standing stocks and fluxes of most components were at similar levels in both regions, the standing stocks of metazooplankton were reported to be ten times greater in the subarctic region than in the subtropical region (Honda et al., 2016(Honda et al., , 2017;;Matsumoto et al., 2016;Wakita et al., 2016).To explain the high mesozooplankton standing stocks, Kitamura et al. (2016) proposed three factors: (i) The respiratory and excretory carbon losses per unit biomass due to large organisms were lower than those due to small organisms (mesozooplankton at the K2 were larger than those at the S1), (ii) the lower temperature at St. K2 allowed the mesozooplankton to decrease respiratory carbon loss and (iii) the higher turnover rates of small organisms at St. S1 may have helped maintain their biomass to low levels.The energy loss from primary producers to the mesozooplankton community at St. K2, in which primary consumers were probably dominant, was considered to be lower than the community at St. S1.The regional patterns of the standing stocks of pelagic polychaetes fit well with the mesozooplankton stocks in each region.The difference in the body lengths of dominant Tomopteris species (without tail) was detected between the subarctic and subtropical regions, and this may have led to variations in the standing stocks between St. S1 and K2 (Kitamura et al., 2016).To mention pelagic polychaetes for other discussions (2) and (3) in Kitamura et al. (2016), we need more information on their life cycle, metabolic rate and food.

Factors affecting community structure
According to the results of multiple regression analysis and PERMANOVA, the most important factor affecting the clustering of the pelagic polychaete community in the subtropical region was depth (P < 0.001, Table V).In the subarctic region, the season was also found to affect the pelagic polychaete community in addition to season (P < 0.05; Amei et al., 2021).This may be related to the seasonal occurrence of the community in which P. meseres was the dominant species, which feeds on sinking particles (detritus) and is abundant in the 100-300 m depths during spring and summer when a large amount of sinking particle detritus is available (Amei et al., 2021).For both regions, interactions related to day or night (×D/N) had a significant effect on the polychaete community (Table V), possibly owing to the effect of DVM of surface-dwelling carnivorous species (e.g.Tomopteris spp.) in each region.
The abundance and biomass of pelagic polychaetes in subtropical S1 were concentrated at 200-300 m occupied with STMW and decreased rapidly at deeper layer occupied with TW and NPIW (Figs. 3 and 4).SPW was only observed in November and July.However, no differences in pelagic polychaete abundance and composition on the surface layer between the seasonal thermocline season (November, July) and non-seasonal thermocline season (February, April) were detected.This suggests that the seasonal thermocline cannot be a barrier to pelagic polychaete distribution at St. S1.The limited distribution of visual carnivore feeders, Alciopini (tribe) (cluster groups A-C) in surface water (SPW and STMW), is related to the vertical changes in the light transmission rate.Among the three groups, the group with the highest DO level had the highest mean abundance (group A > B > C) (Fig. 6c).Carnivores such as Tomopteris and Alciopini (tribe) dominate the surface group and have high metabolic rates for DVM, positive swimming and seeks prey (Childress and Thuesen, 1993;Thuesen and Childress, 1993).Besides, Joost et al. (2021) suggested that Tomopteridae need to stroke their parapodium when they swim actively.Thus, they can need high concentrations of DO to feed on their prey actively.
Poeobius meseres has unique feeding modes wherein it secretes mucus nets to capture the sinking food particles (detritus) (Robison et al., 2010;Uttal and Buck, 1996); this species also has a low metabolic rate and engages in little swimming activity, which may be considered adaptations to the low oxygen concentration in the deep layer, i.e. the OML (Childress and Thuesen, 1993;Thuesen and Childress, 1993).In the subarctic and subtropical regions, OML, which is a prominent environmental parameter for vertical changes, is observed below 200 m (Fig. 2).From the viewpoint of adaptation to the OML, the low metabolic rates of P. meseres (Childress and Thuesen, 1993;Thuesen and Childress, 1993) may facilitate its distribution in the OML.Notably, P. meseres occurs only in the deep layer of the subarctic region (Amei et al., 2021).Because the vertical flux of fecal pellets is reported to be much higher in the subarctic region than in the subtropical region (Kobari et al., 2016), the increased food availability may also support the distribution of P. meseres in the subarctic region.In the pelagic polychaete community, carnivorous pelagic polychaetes dominated the surface of both the subarctic and subtropical regions.However, the compositions of deep-sea communities varied regionally.The scarcity of sinking particles and the low availability of fecal pellet flux in the subtropical region (Kobari et al., 2016) may prevent the distribution of particle feeders in the deep layer of the subtropical region.In the deep layer of the subarctic region, a unique, specialized community of pelagic polychaetes adapted to low oxygen conditions is supported by the abundant nutrient flux via sinking particles and zooplankton fecal pellets.Thus, the two regions differed in the pelagic polychaete community structures and the vertical patterns.

CONCLUSIONS
This study was conducted for latitudinal comparison with the results of Amei et al. (2021), which revealed diel, seasonal, and vertical changes in the pelagic polychaete community at St. K2 in the western subarctic Pacific.By applying the same sampling and analytical methods as Amei et al. (2021) at St. S1 in the western subtropical Pacific, we evaluated several latitudinal differences in the pelagic polychaete community.First, the numbers of genera and species were higher at St. S1 than at St. K2.The annual mean abundance and biomass were much higher at St. K2.These regional patterns were attributed to regional differences in productivity, temperature-induced metabolism and community structure at each region.At St. S1, carnivorous pelagic polychaetes were dominant throughout the sampling layers and concentrated in the surface layer, and their abundance rapidly decreased with increasing depth.On the other hand, at St. K2, numerous particle-feeding species were present in the pelagic polychaete community, and their abundance peaked at a depth of 300 m, with the predominance of particle feeders at the deep layer of the station; this finding can be attributed to the passive flux of sinking particulate matter, such as fecal pellets, at St. K2.The pelagic polychaete community structures varied depending on depth, and they differed between the two regions.

Fig. 1 .
Fig. 1.Location of the sampling stations: St. S1 (30 • N, 145 • E) in the subtropical western North Pacific Ocean.For comparison, the location of the subarctic station (St.K2) is also shown.Approximate positions of the currents are superimposed (cf.Yasuda, 2003).

Fig. 2 .
Fig. 2. Vertical changes in temperature (T), salinity (S), DO (O 2 ), and fluorescence (F) at St. S1 during the sampling period from November 2010 to July 2011.The dotted guidelines show the mean value of all data for each parameter (T:11.00• C, F: 0.097, DO: 3.34 mL L −1 , S: 34.4).Circled numbers in the right column indicate the depth strata of the zooplankton sampling in this study.

Fig. 3 .
Fig. 3. T-S diagram for the 0-1000 m water column at St. S1 during November 2010, February, April and July 2011.Five water masses were identified: SPW, STMW, TW, Upper NPIW and Lower NPIW.Solid lines on the background are isopycnals.Differences in the colors of the symbols denote differences in the zooplankton sampling layers.

Fig. 4 .
Fig. 4. Vertical changes in polychaete abundance (ind.1000 m −3 ), species composition, and species diversity (H ) in the 0-1000 m water column at St. S1 from November 2010 to July 2011.The composition of each genus is shown with a different color.

Fig. 5 .
Fig. 5. Vertical changes in polychaete biomass (wet weight: mg WW 1000 m −3 ), species composition and species diversity (H ) in the 0-1000 m water column at St. S1 from November 2010 to July 2011.The composition of each genus is shown with a different color.

Fig. 6 .
Fig. 6.(a) Results of cluster analysis based on pelagic polychaete abundance at St. S1.Six groups (A-F) were identified at 45% Bray-Curtis dissimilarity connected with the complete linkage method.Numbers in parentheses indicate the number of samples each group contained.(b) NMDS plots of each group.Arrows indicate directions of significant environmental parameters.Dep.: depth, Temp.: temperature, Sal.: salinity, DO: dissolved oxygen, Fluo.: fluorescence.Stress is a value used to evaluate the quality of NMDS, which is low for relevant NMDS.(c) Mean abundance (white circle) and genus composition of each group.

Fig. 7 .
Fig. 7. Seasonal, vertical, and diel changes in the occurrence of the five polychaete community groups (A-F) identified by Bray-Curtis dissimilarity based on their abundance at St. S1 from November 2010 to July 2011 (cf.Fig. 6a).D: day, N: night.

Fig. 9 .
Fig. 9. Vertical changes in the polychaete abundance, biomass, species diversity (H ) and genus composition in annual mean abundance (%) for the 0-1000 m water column in the subarctic (St.K2) and the subtropical (St.S1) regions in western North Pacific.Lines represent values for days and nights during the four seasons from 2010 to 2011.

Fig. 10 .
Fig. 10.Percentage composition of sampling layers in pelagic polychaete distribution within 0-1 000 m water column reported by this study and previous studies.Study No. corresponds to Table VI.No. 1-5 research in the subtropical region and the rest in the subarctic region of the Pacific Ocean.

Table I :
Zooplankton sampling data (eight vertical stratification samplings between 0 and 1000 m) at St. S1 in subtropical western North Pacific from November 2010 to July 2011.D: day, N: night

Table II :
List of the pelagic polychaete species identified in the 0-1000 m water column at St. S1 in the western subtropical North Pacific from November 2010 to July 2011.Annual mean abundance and biomass (mean ± 1 SE: ind.1000 m −3 or mg WW 1000 m −3 at 0-1000 m water column), species composition (%) and depth ranges (m) of occurrence are shown

Table III :
Comparison of polychaete abundances for the five water masses (SPW, STMW, TW, Upper NPIW, Lower NPIW) at St. S1.Mean abundances for each water mass are shown.Differences between water masses were tested by one-way ANOVA and Tukey-Kramer test.For the results of the Tukey-Kramer test, differences in superscript letters indicate significant differences (P < 0.05).Numbers in the parentheses indicate the number of samples included in each water mass • N * P < 0.05; * * * P < 0.001; NS: not significant; Alphabets on values show group.
. Mean abundances for each group are shown.Differences between groups were tested by one-way ANOVA and Tukey-Kramer test.For the results of the Tukey-Kramer test, differences in superscript letters indicate significant differences (P < 0.05).Numbers in the parentheses indicate number of samples included in each group * * * P < 0.001; * P < 0.05; NS: not significant; Alphabets show group.

Table V :
PERMANOVA results based on Bray-Curtis dissimilarities using the polychaete abundance data in relation to the environmental factors at St. S1 (season, day/night and depth).D: day, N: night

Table VI :
Diel-mean abundances of pelagic polychaetes for 0-1000 m depth reported by this study and comparable previous studies conducting vertically stratified sampling into ≥1000 m depth in the subtropical and subarctic Pacific Ocean